This application claims benefit of priority under 35 USC 119 based on Japanese Patent Application No. P2000-199437 filed on Jun. 30, 2000, the entire contents of which are incorporated by reference herein.
1. Field of the Invention
The present invention relates to an FM (frequency-modulated) signal oscillator circuit and a modulation level control method, and more particularly, to a FM signal oscillator circuit that has a voltage-controlled oscillator including a resonator that employs a variable capacitance diode to provide an FM signal.
2. Description of Related Art
The FM signal oscillator circuit employing a voltage-controlled oscillator receives a control voltage to set an oscillation frequency of the voltage-controlled oscillator and a modulating signal to minutely change the control voltage, and provides an FM signal. When modulating the oscillation frequency by changing the control voltage, the FM signal oscillator circuit must keep a modulation level specified by radio regulations.
FIG. 1 is a circuit diagram showing an FM signal oscillator circuit according to a related art. This circuit includes a voltage-controlled oscillator having a resonator that employs a variable capacitance diode to provide an FM signal. More precisely, the FM signal oscillator circuit has a driver 1, the resonator 2, and resistors 8 and 9 to supply a control voltage Vc and a modulating signal Vm to the resonator 2.
The resonator 2 is an LC resonator including the variable capacitance diode 21, a capacitor 22, a coil 23, a capacitor 24, and a resistor 25. The capacitor 24 and resistor 25 form a parallel circuit. The driver 1 and resonator 2 form the voltage-controlled oscillator.
The control voltage Vc is supplied to a frequency control input terminal 4 from which to the resistor 8 and then to a cathode of the diode 21. The control voltage Vc determines a capacitance of the diode 21, and thus, a resonance frequency of the resonator 2 or an oscillation frequency fo of the voltage-controlled oscillator composed of the driver 1 and resonator 2. An output terminal 6 provides an oscillating output signal.
The modulating signal Vm is supplied to an input terminal 5 from which to the resistor 9 and then to an anode of the diode 21. In response to the modulating signal Vm, the capacitance of the diode 21 slightly changes to change the resonance frequency of the resonator 2 around fo, thereby provide an FM signal from the output terminal 6.
According to the related art, the variable capacitance diode 21 of the resonator 2 must have a hyper abrupt junction, so that the voltage-controlled oscillator may maintain a constant modulation level even if the control voltage Vc is changed. Namely, the oscillation frequency fo must be changed by changing the control voltage Vc without fluctuating a modulation level.
FIG. 2 is a graph showing the control voltage (Vc)-oscillation frequency (fo) characteristics of variable capacitance diodes with graded junction (I), abrupt junction (II), and hyper abrupt junction (III). The hyper-abruptjunction diode (III) shows linear Vc-fo characteristics to maintain a constant modulation level even if the control voltage Vc is changed to vary the oscillation frequency fo. The graded-junction diode (I) and abrupt-junction diode (II) show nonlinear Vc-fo characteristics to alter a modulation level when the control voltage Vc is changed to change the oscillation frequency fo.
A problem of the FM signal oscillator circuit employing the hyper-abrupt-junction diode is that it is unable to produce through standard IC (integrated circuit) manufacturing processes because the standard processes are only capable of forming abrupt- or graded-junction diodes on ICs. To form hyper-abruptjunction diodes on ICs, additional manufacturing processes must be involved to increase the cost of the ICs.
The present invention is to provide an FM signal oscillator circuit and a modulation level control method, capable of employing abrupt- or graded junction variable capacitance diodes for a resonator without changing a modulation level, so that the circuit may be produced as an IC through standard IC manufacturing processes.
An FM signal oscillator circuit includes a resonator having a variable capacitance diode whose capacitance is set according to a control voltage, the variable capacitance diode changing a resonance frequency of the resonator, a driver configured to drive the resonator and providing an FM signal, a corrector configured to correct the level of a modulating signal according to resonance-frequency-changing characteristics of the resonator and applying a signal corresponding to the corrected modulating signal and a signal corresponding to the control voltage to the variable capacitance diode.
The FM signal oscillator circuit according to this invention enables to employ graded- or abrupt-junction variable capacitance diodes to fabricate a resonator of the FM signal oscillator circuit that is capable of maintaining a constant modulation level when an oscillation frequency is changed. Due to the graded- or abrupt-junction diodes, this oscillator circuit is manufacturable through standard IC manufacturing processes at low cost.